In various types of high-precision equipment, constituent components must operate at extremely high performance levels to achieve the specified tolerances of the equipment. An example of such equipment is any of various high-performance optical systems (e.g., astronomical telescopes, space-based optical systems, high-power laser systems, microlithography equipment, electron-beam systems, and inspection equipment), high-precision tools and measurement equipment, and particle accelerators. In an optical system, the constituent optical elements such as lenses, filters, and/or mirrors are impinged with the radiation used with the system. If an optical element absorbs some of the incident radiation and especially if the incident radiation is intense, the element likely will experience substantial heating, which can be detrimental to element and/or system performance. For example, an excessive rise in temperature can thermally distort an optical element, thereby degrading its optical performance. The same applies to stages (e.g., reticle stages and substrate stages in microlithography systems, target stages in particle accelerators, specimen stages in electron microscopy systems) used for moving and positioning things relative to the radiation.
An example of a modern high-precision system is a microlithography system. Microlithography systems include mechanical and optical components that must operate at extremely high levels of performance. Most current microlithography tools use wavelengths of deep ultraviolet (DUV) light (k=150 to 250 nm) for imaging purposes. To achieve further improvement of imaging resolution, substantial research currently is being directed to the development of a practical “extreme ultraviolet” (EUV) microlithography system that utilizes an exposure wavelength in the range of 11 to 14 nm. EUV optical systems are entirely reflective and comprise a plurality of mirrors each having a multilayer EUV-reflective coating on its reflective surface to provide the mirror with a usable reflectivity (approximately 70%, maximum) to EUV radiation at non-grazing angles of incidence.
The mirrors in various high-precision optical systems often require cooling to maintain performance stability. The need for cooling is particularly acute with the mirrors of the illumination unit of a modern EUV microlithography tool, for example. EUV sources are very intense and radiate a large amount of energy (of which a small fraction is actually usable for lithographic exposure), and efforts are ongoing to increase their intensity even further. EUV-reflective mirrors, particularly in the illumination unit, are vulnerable to substantial heating during use because, inter alia, they are located relative close to the EUV source, and their multilayer reflective coatings absorb a substantial amount (with the current best mirrors, approximately 30%) of incident EUV radiation. In the illumination unit the mirror situated closest to the source receives the most illumination energy, up to 1 kW or greater. Downstream mirrors receive correspondingly less radiant energy. If inadequately cooled, the mirror can experience thermal effects (e.g., warping) that can cause an unacceptable degradation of optical performance of and possible fracture or other damage to the mirror.
Most microlithography systems also have other components such as stages and chucks whose operating temperatures must remain within tight limits for optimal performance. An example stage is a reticle stage, which typically includes a reticle chuck. During use the reticle stage and chuck are in-line to receive substantial radiation that can cause heating, in addition to the energy absorbed by the reticle itself. The reticle stage also includes actuators, sensors, and the like that generate heat. This heat usually should be removed to obtain optimal performance of the reticle stage.
One conventional approach to component cooling is passive cooling achieved by, for example, placing the component in contact with a large heat-sink or the like. Unfortunately, this approach is often not effective, particularly if the component is being heated rapidly or is being heated at a variable rate.
Another approach involves circulating water or refrigerant through conduits in the component and/or in a heat sink contacting the component. See, for example, U.S. patent application Ser. No. 12/001,529, filed on Dec. 11, 2007, and incorporated herein by reference. This manner of cooling provides increased rates of heat removal from the component compared to passive cooling. However, under rapid-heating conditions, these coolants may not remove heat sufficiently rapidly at practical liquid flow-rates. Also, whereas increasing the flow-rate of liquid through the conduits increases the rate of heat transfer from the component to the liquid, increased flow-rates are generally more turbulent, which produces vibrations. Also, the conventional manner of routing the coolant liquid to and from the component using external hoses or the like connected to the component usually results in increased vibrations being transmitted to the component.
A conventional cooling scheme is shown in FIG. 10, depicting a component 10 (e.g., mirror) and an external coolant (e.g., water) pump 12. The mirror 10 is mounted to a frame 14 by vibration-isolation or -attenuating mounts 16 that inhibit transmission of vibrations from the frame 14 to the mirror 10. The mirror 10 includes an incidence surface 18 and a body 20. The incidence surface 18 receives heat (arrows 22) from radiation impinging thereon. A coolant channel 24 extends through the body 20, and the pump 12 is connected to the coolant channel 24 by external hoses 26. As the radiation 22 impinges on the surface 18, the surface absorbs some of the incident radiation energy as heat. The heat is conducted through the body 20 to the channel 24 and is transferred to the liquid in the channel. During use of the mirror 10 the pump 12 circulates the liquid through the hoses 26 and coolant channel 24. The hoses 26 are also connected to an external heat-exchanger 28 (e.g., temperature-regulated liquid-cooling device) to remove heat from the liquid before the liquid is returned to the body 20. This conventional cooling scheme is effective for some applications, but is not entirely satisfactory for others, especially applications in which the rate of heat absorption by the mirror is greater than the rate at which heat can be removed by the liquid and/or applications that are too sensitive to vibrations produced by the circulating coolant.
Therefore, a need exists for methods and devices for cooling vibration-sensitive optical and other components in a manner that: (a) inhibits internal generation of vibrations, (b) inhibits transmission of external vibrations to the components, and (c) achieves a satisfactory rate of heat removal.